The Most Terrifying Prehistoric Creatures That Roamed the Vast Seas

These are the most fearsome prehistoric creatures that terrorized the depths of the ocean millions of years ago.

Isabel Cara

Paleontology has advanced by leaps and bounds, and thanks to that, we can now have a clearer idea of what the beings that inhabited our planet in ancient times were like. Paleontologists have found traces not only of land creatures but also of the prehistoric creatures that sailed in the depths of the ocean. Of colossal sizes, these are the most fearsome prehistoric sea creatures.

Fearsome Prehistoric Marine Animals


It lived in the middle Eocene, which occurred between 46 and 77 million years ago. Ranging in size from 1.5 to 5 meters in length, Rodhocetus is a clear example of the evolutionary transition between a terrestrial and a marine creature, as it was a cetacean similar to a walrus but with large hind limbs that it retained for its terrestrial gait. It was a creature that was adaptive to the sea but still retained some terrestrial characteristics, just as walruses do today.

The Rodhocetus fossil finds suggest that it lived in the regions we know today as Egypt and Pakistan. In addition, they have provided valuable information on the early evolution of whales, which, let us remember, are cetaceans as well.

Terrifying sea prehistoric creatures3 - the most terrifying prehistoric creatures that roamed the vast seas


They were the group of marine reptiles closest to what we know today as dolphins, although their sizes were much larger than today’s. They lived from the Lower Triassic to the Late Cretaceous. They lived from the Lower Triassic to the Upper Cretaceous, that is, they inhabited the Earth for about 150 million years. Their bodies were completely adapted to marine life, they had long snouts and four gigantic flippers, in fact, the largest fossils of Ichthyosaurs suggest that they reached up to 20 meters in length. They became extinct about 90 million years ago and now we only know about them thanks to fossils found in sediments that were once covered by the ocean.

Terrifying sea prehistoric creatures2 - the most terrifying prehistoric creatures that roamed the vast seas


Mosasaurs are the epitome of fearsome creatures that inhabited the ocean. They were a group of marine reptiles that devoured almost everything in their path, including other smaller mosasaurs. They are considered to be sea monsters as they reached up to 18 meters in length and possessed jaws with numerous sharp teeth to tear their victims apart. They were the most savage prehistoric marine predators of the Cretaceous, some 70-66 million years ago.

Terrifying sea prehistoric creatures4 - the most terrifying prehistoric creatures that roamed the vast seas


So far we have talked about gigantic reptiles that navigated in the depths, but the prehistoric ocean was also inhabited by other strange creatures such as the Eurypterids, a class of arthropods that distinguishes itself from all others by having reached gigantic sizes for its species. They were similar in appearance to modern-day land scorpions, except that they were aquatic and fossils suggest that they reached up to 2.5 meters. Imagine a sea scorpion of such size, simply amazing.

These creatures are perhaps the oldest on the list, living in the Paleozoic era 467 million years ago. Eurypterids inhabited the Earth for about 200 million years and then became extinct at the end of the Permian period.

Terrifying sea prehistoric creatures1 - the most terrifying prehistoric creatures that roamed the vast seas

Story written in Spanish by Alejandra Martínez in Ecoosfera

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